Depleting intracellular putrescine and spermidine with difluoromethylornithine enhanced the susceptibility of the cellular DNA to amsacrine-induced protein-associated DNA strand breaks. Exogenous putrescine reversed this effect. Arabinosylcytosine, hydroxyurea and 5-azacytidine enhanced the cytotoxic and DNA breaking activities of amsacrine in murine L1210 cells. These changes appear to be mediated by alterations in the distribution of cells within the cell cycle and/or alterations in DNA methylation. A correlation between the DNA double-strand breakage produced by amsacrine or 5-iminodaunorubicin and the cytotoxicity, mutagenicity and sister-chromatid exchange produced by the 2 intercalators was found. Single-strand scission did not correlate with these biological parameters.